A number of processes have have been reported for the preparation of aromatic mono- and polyisocyanates by the vapor or solvent phase decomposition of substituted ureas.
The production of aromatic isocyanates from symmetrical bis aryl ureas in the presence of hydrogen chloride, phosphorus pentoxide or zinc chloride was described by A. Hofmann in the Proc. Royal Soc., London, Vol. IX, p. 274 (1858). By heating a mixture of diphenyl urea with phosphorus pentoxide, zinc chloride or gaseous HCl; Hofmann distilled phenyl isocyanate overhead. No details of the experimental procedure are presented and the yield of isocyanate is not given.
A. Hofmann, Chemisch Berichte, Vol. 3, pp. 653-658 (1870) described heating diphenyl urea in the presence of phosphorus pentoxide giving yields too small to be considered for the preparation of the isocyanate.
Subsequent work by Iwakura and Nagakubo reported in the Bulletin Tokyo Inst. Technol., Vol. 13, p. 25 (1950) and Chemical Abstracts, Vol.44, p. 3924e (1950) describes the preparation of an aromatic isocyanate (p-ethoxyphenylisocyanate) by heating a solution of bis aryl urea such as bis (p-ethoxyphenyl) urea in the presence of hydrogen chloride gas.
The vapor phase decomposition of bis aryl ureas at 350.degree. C. and higher temperatures has been described by W. D. Bennet et al, Journ. Am. Chem. Soc., Vol. 75, p. 2101 (1952) and Slocombe et al in U.S. Pat. No. 2,773,086, Dec. 4, 1956 in the presence of gaseous HCl as a promoter. Yields are reported in the 60 to 70% range for the vapor phase reaction and only a 5% yield for liquid phase reaction. A carbamoyl chloride intermediate is formed.
The liquid phase decomposition of trisubstituted ureas to isocyanates has been described by van Landeghem et al, French Patent No. 1,473,821, Feb. 13, 1967; C. J. Hearsey, U.S. Pat. No. 3,898,259, Aug. 5, 1975 and Rosenthal et al in the U.S. Pat. No. 3,936,484, Feb. 3, 1976. van Landeghem shows thermal decomposition of trisubstituted ureas in an organic solvent having specified dielectric constants at 140.degree. to 170.degree. C. with long reaction times of from 6 to 10 hours and modest yields of 60 to 75%. A variety of catalysts are shown but not exemplified or claimed, and include metal salts, such as acetates, stearates, and linoleates of manganese, zinc, cobalt, chromium and vanadium, tertiary amine bases, such as aliphatic, cycloaliphatic, aromatic and mixed tertiary amines, aliphatic heterocyclic amines such as N-methylpiperidine or N, N'-dimethylpiperidine as well as aromatic heterocyclic amines such as pyridine and pyrimidine. Other nitrogen compounds such as imidazole are indicated as being suitable. However, under the reaction conditions described tertiary amines as shown by van Landeghem do not catalyze urea decomposition.
Rosenthal et al U.S. Pat. No. 3,936,484 discloses the thermal decomposition of di- and tri-substituted ureas to isocyanates at temperatures above 230.degree. C. in a solvent and isocyanate yields of from 60 to 80%.
The Hearsey U.S. Pat. No. 3,898,259 describes the introduction of gaseous hydrogen chloride into the liquid phase urea decomposition reaction to give reduced reaction times with isocyanate yeilds of from 8-90%. An excess of gaseous HCl is employed and a by-product carbamoyl chloride intermidiate formed.
A. Hentschel et al U.S. Pat. No. 4,223,145, Sept. 16, 1980 discloses the formation of an HCl adduct of a trisubstituted urea using at most a 10% excess of HCl. This adduct is then decomposed in a closed system at from 80.degree.-180.degree. C.
Applicants have found that hydrohalide salts of tertiary amines are very effective promoters for the thermal decomposition of trisubstituted ureas to the corresponding isocyanates at relatively mild reaction temperatures using short residence times as an organic solvent. Although it has been shown that the hydrohalide acids (HCl) promote urea decomposition, it is unexpected that the salts of these acids, especially, tertiaryl amine salts of these acids would be effective promoters. The isocyanates obtained by the instant process are in obtained in nearly quantitative yield.